Photoreduced graphene oxide recovers graphene hot electron cooling dynamics

Reduced graphene oxide (rGO) is a bulk-processable quasiamorphous two-dimensional material with broad spectral coverage and fast electronic response. rGO sheets are suspended in a polymer matrix and sequentially photoreduced to measure the optical absorption and ultrafast hot-electron cooling dynamics. Photoreduced graphene oxide (GO) yields absorption spectra that fit the same Fano line shape parameters as monolayer (ml) graphene. With increasing photoreduction time, rGO transient absorption kinetics reach an optimal point that matches the hot-electron cooling dynamics simultaneously measured in ml-graphene. After each stepwise photoreduction, the rGO ultrafast kinetics are simulated with the hot-electron cooling model of graphene that is mediated by disorder-assisted supercollisions. The hot-electron cooling rate of moderately photoreduced GO is 0.31 ps(-1) and closely matches the ml-graphene result. Subsequent photoreduction increases the disorder parameter associated with the hot-election cooling, consistent with an order of magnitude smaller mean-free scattering length from photoionized point defects. GO photoreduction yields increasing photoluminescence from localized graphene quantum dot (GQDs) and decreasing 2.3 eV emission from oxygenated defect-edge states. By pumping localized GQD states and probing graphene, a 0.17 ps delayed rise-time emerges that accelerates with photoreduction, suggesting an energy transfer process.